The check engine light came on in my 94 Ford Escort after having transmission work done. The trouble codes are 335,326 and 172, and supposedly relate to the DPFE sensor. I installed a new sensor, but the light is still on. Does the computer reset right away, or is there something else wrong?

The Check Engine Light Code(s) must be cleared. On this vintage Ford Escort you can disconnect the Negative Battery Cable for a minute or so and that should clear them if there is no other problem. The code 172 is usually an Oxygen Sensor 'not switching code' though, so the DPFE Sesnor will not help that.